TMR Device with Novel Free Layer Structure

ABSTRACT

A method of fabricating a TMR sensor that includes a free layer having at least one B-containing (BC) layer made of CoFeB, CoFeBM, CoB, CoBM, or CoBLM, and a plurality of non-B containing (NBC) layers made of CoFe, CoFeM, or CoFeLM is disclosed where L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb. In every embodiment, a NBC layer contacts the tunnel barrier and NBC layers each with a thickness from 2 to 8 Angstroms are formed in alternating fashion with one or more BC layers each 10 to 80 Angstroms thick. Total free layer thickness is &lt;100 Angstroms. The TMR sensor may be annealed with a one step or two step process. The free layer configuration described herein enables a significant noise reduction (SNR enhancement) while realizing a high TMR ratio, low magnetostriction, low RA, and low Hc values.

This is a Divisional application of U.S. patent application Ser. No.12/284,409, filed on Sep. 22, 2008, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. No. 7,780,820;U.S. Pat. No. 8,472,151; and U.S. Pat. No. 9,021,685; all assigned to acommon assignee and herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a high performance tunneling magnetoresistive(TMR) sensor in a read head and a method for making the same, and inparticular, to a composite free layer comprised of multiple CoFe or CoFealloy layers alternating with one or more CoB, CoB alloy, CoFeB, orCoFeB alloy layers to reduce head noise while achieving acceptable RA(resistance×area) and dR/R values.

BACKGROUND OF THE INVENTION

A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is akey component (memory element) in magnetic devices such as MagneticRandom Access Memory (MRAM) and a magnetic read head. A TMR sensortypically has a stack of layers with a configuration in which twoferromagnetic layers are separated by a thin non-magnetic insulatorlayer. The sensor stack in a so-called bottom spin valve configurationis generally comprised of a seed (buffer) layer, anti-ferromagnetic(AFM) layer, pinned layer, tunnel barrier layer, free layer, and cappinglayer that are sequentially formed on a substrate. The free layer servesas a sensing layer that responds to external fields (media field) whilethe pinned layer is relatively fixed and functions as a reference layer.The electrical resistance through the tunnel barrier layer (insulatorlayer) varies with the relative orientation of the free layer momentcompared with the reference layer moment and thereby converts magneticsignals into electrical signals. In a magnetic read head, the TMR sensoris formed between a bottom shield and a top shield. When a sense currentis passed from the top shield to the bottom shield (or top conductor tobottom conductor in a MRAM device) in a direction perpendicular to theplanes of the TMR layers (CPP designation), a lower resistance isdetected when the magnetization directions of the free and referencelayers are in a parallel state (“1” memory state) and a higherresistance is noted when they are in an anti-parallel state or “0”memory state. Alternatively, a TMR sensor may be configured as a currentin plane (CIP) structure that indicates the direction of the sensecurrent.

A giant magnetoresistive (GMR) head is another type of memory device. Inthis design, the insulator layer between the pinned layer and free layerin the TMR stack is replaced by a non-magnetic conductive layer such ascopper.

In the TMR stack, the pinned layer may have a syntheticanti-ferromagnetic (SyAF) configuration in which an outer pinned layeris magnetically coupled through a coupling layer to an inner pinnedlayer that contacts the tunnel barrier. The outer pinned layer has amagnetic moment that is fixed in a certain direction by exchangecoupling with the adjacent AFM layer which is magnetized in the samedirection. The tunnel barrier layer is so thin that a current through itcan be established by quantum mechanical tunneling of conductionelectrons.

A TMR sensor is currently the most promising candidate for replacing aGMR sensor in upcoming generations of magnetic recording heads. Anadvanced TMR sensor may have a cross-sectional area of about 0.1micron×0.1 micron at the air bearing surface (ABS) plane of the readhead. The advantage of a TMR sensor is that a substantially higher MRratio can be realized than for a GMR sensor. In addition to a high MRratio, a high performance TMR sensor requires a low areal resistance RA(area×resistance) value, a free layer with low magnetostriction (λ) andlow coercivity (Hc), a strong pinned layer, and low interlayer coupling(Hin) through the barrier layer. The MR ratio (also referred to as TMRratio) is dR/R where R is the minimum resistance of the TMR sensor anddR is the change in resistance observed by changing the magnetic stateof the free layer. A higher dR/R improves the readout speed. For highrecording density or high frequency applications, RA must be reduced toabout 1 to 3 ohm-um².

A MgO based MTJ is a very promising candidate for high frequencyrecording applications because its tunneling magnetoresistive (TMR)ratio is significantly higher than for AlOx or TiOx based MTJs. S. Yuasaet al. in “Giant room-temperature magnetoresistance in single crystalFe/MgO/Fe magnetic tunnel junctions”, Nature Materials 3, 868-871 (2004)and S. Parkin et al. in “Giant tunneling magnetoresistance at roomtemperature with MgO (100) tunnel barriers”, Nature Materials 3, 862-867(2004) demonstrated that a MR ratio of ˜200% can be achieved at roomtemperature in epitaxial Fe(001)/MgO(001)/Fe(001) and in polycrystallineFeCo(001)/MgO(001)/(Fe₇₀CO₃O₈₀B₂₀ MTJs. Yuasa et al. reported an MRratio as high as 410% at RT in “Giant tunneling magnetoresistance up to410% at room temperature in fully epitaxial Co/MgO/Co magnetic tunneljunctions with bcc Co(001) electrodes”, Applied Physics Letters, 89,042505 (2006). Meanwhile, D. Djayaprawira et. al showed that MTJs ofCoFeB/MgO(001)/CoFeB structure made by conventional sputtering can alsohave a very high MR ratio of 230% with advantages of better flexibilityand uniformity in “230% room temperature magnetoresistance inCoFeB/MgO/CoFeB magnetic tunnel junctions”, Physics Letters 86, 092502(2005).

For a low RA application, the MR ratio of CoFeB/Mg/MgO/CoFeB MTJs canreach 138% at RA=2.4 ohm/μm² according to K. Tsunekawa et al. in “Gianttunneling magnetoresistance effect in low resistanceCoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read headapplications”, Applied Physics Letters 87, 072503 (2005). In this case,a DC-sputtered Mg layer was inserted between the CoFeB pinned layer andan RF-sputtered MgO layer, an idea initially proposed by T. Linn et al.in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode(CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure. Also, a Tagetter presputtering prior to RF sputtering a MgO layer can achieve 55%TMR with 0.4 ohm/μm² as reported by Y. Nagamine et al. in “Ultralowresistance-area product of 0.4 ohm/μm² and high magnetoresistance above50% in CoFeB/MgO/CoFeB magnetic junctions”, Applied Physics Letters, 89,162507 (2006).

An alternative method of forming low RA in a MTJ with a MgO tunnelbarrier is to DC sputter deposit a first Mg layer, perform a naturaloxidation (NOX) process, and then DC sputter deposit a second Mg layeron the resulting MgO layer as disclosed in related U.S. Pat. No.7,780,820. Benefits include better process control and improved MRR(read) uniformity.

In order to achieve a smaller He but still maintain a high TMR ratio,the industry tends to use CoFeB as the free layer in a TMR sensor basedon a MgO tunnel barrier. Insertion of a thin CoFe layer between a MgObarrier and CoFeB free layer may be used for realizing a high MR ratioat a low annealing temperature below 300° C. However, there are twomajor concerns associated with a CoFeB free layer. One is a highpositive magnetostriction (λ) and a second issue is a CoFeB free layertends to cause excessive noise and lower the signal to noise ratio (SNR)which is undesirable. Thus, an improved free layer in a TMR sensor isneeded that reduces noise and magnetostriction while providing a highTMR ratio, low RA value, and low coercivity.

U.S. Pat. No. 7,310,210 mentions a CoFeB/Cu/CoFeB free layer where thelarger spin polarization on the boundaries between the CoFeB layers andCu layer promote spin dependent scattering and enhance themagnetoresistive effect.

In U.S. Pat. No. 6,982,932, a free layer is disclosed which is alaminate of CoFeB and CoNbZr. The laminate may be formed on a CoFe layerto provide an interface between the free layer and an isolating layer.

U.S. Patent Application Publication No. 2007/0253116 describes amagnetic layer that contains CoFe, CoFeB, a CoFe alloy, or a combinationof these films.

U.S. Patent Application Publication No. 2005/0052793 teaches a freelayer with a trilayer configuration wherein each of the first, second,and third layers is selected from a group including Ni, Co, Fe, B, CoFe,CoFeB, NiFe, and alloys thereof.

U.S. Patent Application Publication No. 2003/0123198 discloses a freelayer made of a CoFe film, NiFe film, or a CoFeB film or a laminationlayer film of these films to realize a larger MR ratio and a softmagnetic characteristic.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a TMR sensor with afree layer composition that reduces noise compared with CoFeB orCoFe/CoFeB while maintaining an acceptable TMR value and achieving amagnetostriction between −5×10⁻⁶ and 5×10⁻⁶, a low RA value below 3ohm-μm², and a low coercivity in the range of 4 to 7 Oe.

A further objective of the present invention is to provide a method offorming a TMR sensor that satisfies the first objective and is costeffective.

According to one embodiment of the present invention, these objectivesare achieved by forming a TMR sensor on a suitable substrate such as abottom shield in a read head. The TMR sensor may have a bottom spinvalve configuration comprised of a seed layer, AFM layer, pinned layer,tunnel barrier layer, free layer, and capping layer which are formedsequentially on the bottom shield. The tunnel barrier layer ispreferably MgO formed by a process involving a natural oxidation of a DCsputter deposited Mg layer. Optional tunnel barrier materials are MgZnO,ZnO, Al₂O₃, TiOx, AITiO, HfOx, ZrOx, or combinations thereof or withMgO. In one embodiment, the free layer is a composite comprised of astack represented by (CoFe/CoFeB)_(n), (CoFeM/CoFeB)_(n),(CoFe/CoFeBM)_(n), (CoFeM/CoFeBM)_(n), (CoFeLM/CoFeB)_(n), or(CoFeLM/CoFeBM)_(n) where M and L are one of Ni, Ta, Ti, W, Zr, Hf, Tb,and Nb, M is unequal to L, and n≧2. There is a second embodiment wherethe free layer is a composite comprised of a stack represented by(CoFe/CoB)_(n), (CoFeM/CoB)_(n), (CoFeLM/CoB)_(n), (CoFe/CoBM)_(n),(CoFeM/CoBM)_(n), (CoFeLM/CoBM)_(n), (CoFe/CoBLM)_(n), (CoFeM/CoBLM, or(CoFeLM/CoBLM)_(n) where L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb,or Nb, M is unequal to L, and n≧2. The CoB layer has a compositionrepresented by Co_(S)B_(T) where T is from 5 to 30 atomic % and S+T=100atomic %. In the first two embodiments, each of the plurality of CoFeB,CoFeBM, CoB, CoBM, or CoBLM layers has a thickness greater than each ofthe plurality of CoFe, CoFeM, or CoFeLM layers.

In a third embodiment, the free layer stack is represented by a(CoFe/CoFeB)_(n)/CoFe, (CoFeM/CoFeB)_(n)/CoFeM, (CoFe/CoFeBM)_(n)/CoFe,(CoFeM/CoFeBM)_(n)/CoFeM, (CoFeLM/CoFeB)_(n)/CoFeLM, or(CoFeLM/CoFeBM)_(n)/CoFeLM configuration where n is ≧1, and L and M areone of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb, and L is unequal to M. The oneor more CoFeB or CoFeBM layers preferably have a greater thickness thaneach of the CoFe, CoFeM, or CoFeLM layers.

There is a fourth embodiment wherein the free layer has a configurationrepresented by (CoFe/CoB)_(n)/CoFe, (CoFe/CoBM)_(n)/CoFe,(CoFe/CoBLM)_(n)/CoFe, (CoFeM/CoB)_(n)/CoFeM, (CoFeM/CoBM)_(n)/CoFeM,(CoFeM/CoBLM)_(n)/CoFeM, (CoFeLM/CoB)_(n)/CoFeLM,(CoFeLM/CoBM)_(n)/CoFeLM, or (CoFeLM/CoBLM)_(n)/CoFeLM where n≧1, L andM are one of Ni, Ta, Ti, W, Zr, Hf, Tb, and Nb, and L is unequal to M.The B content in the CoB, CoBM, or CoBLM alloy is from about 5 to 30atomic %. The one or more CoB, CoBM, or CoBLM layers preferably have agreater thickness than each of the CoFe, CoFeM, or CoFeLM layers.

Typically, a TMR stack of layers is laid down in a sputtering system.All of the layers may be deposited in the same sputter chamber.Preferably, the MgO tunnel barrier is formed by depositing a first Mglayer on the pinned layer followed by a natural oxidation process on thefirst Mg layer to form a MgO layer and then depositing a second Mg layeron the MgO layer. The oxidation step is performed in an oxidationchamber within the sputtering system. The TMR stack is patterned by aconventional method prior to forming a top shield on the cap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a TMR stack of layers accordingto one embodiment of the present invention.

FIG. 2 is an enlarged drawing of the free layer in FIG. 1 thatillustrates the various layers within the composite structure accordingto one embodiment of the present invention.

FIG. 3 is an enlargement of the free layer in FIG. 1 that shows thelayers within the composite structure according to a second embodimentof the present invention.

FIG. 4 is a cross-sectional view showing a TMR stack of layers that hasbeen patterned to form a MTJ element during an intermediate step offabricating the TMR sensor according to one embodiment of the presentinvention.

FIG. 5 is a cross-sectional view of a TMR read head having a MTJ elementinterposed between a top shield and bottom shield and formed accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a high performance TMR sensor featuring acomposite free layer comprised of CoFeB, CoB, or alloys thereof in whichexcessive noise normally associated with these boron containing magneticmaterials is reduced by forming a free layer configuration that includesa plurality of CoFe or CoFe alloy layers. While the exemplary embodimentdepicts a TMR sensor in a read head, the present invention may beemployed in other devices based on a tunneling magnetoresistive elementsuch as CIP-GMR or CPP-GMR sensors. Although a bottom spin valvestructure is described for a TMR sensor, the present invention alsoencompasses a top spin valve or multilayer spin valve configuration asappreciated by those skilled in the art. Drawings are provided by way ofexample and are not intended to limit the scope of the invention. Forexample, various elements are not necessarily drawn to scale and theirrelative sizes may differ compared with those in an actual device.

Previously, the inventors have practiced a method in related U.S. Pat.No. 9,021,685 whereby a two step annealing procedure is used to restoresome of the magnetic softness lost when a CoFeB layer is included in afree layer. The procedure involves applying a magnetic field with afirst temperature and for a first length of time and then applying amagnetic field with a second temperature greater than the firsttemperature but for less than the first length of time. However, noisereduction is not achieved by this procedure.

It should be understood that in a CoFeB based free layer, a certain Bcontent is needed to achieve the required magnetic softness. However, alarge amount of non-magnetic B (typically 20 atomic %) that increasessoftness also tends to introduce extra noise and cause a degradation inthe signal-to-noise ratio (SNR) in the read head. Higher B content alsoleads to high magnetostriction (λ) in the device which is a concern.Related U.S. Pat. No. 8,472,151 discloses how a CoFeB composition may beadjusted to reduce λ and how CoB with a slightly negative λ value may beused to replace CoFeB with a large positive λ value. However, there isstill a significant noise associated with a CoB layer because of itsboron content. Therefore, we were motivated to further modify the freelayer to improve performance and have discovered a composite free layerstructure containing boron that reduces noise (improves SNR) whilerealizing a high dR/R, low λ value, low RA, and low coercivity.

Referring to FIG. 1, a portion of a partially formed TMR sensor 1 of thepresent invention is shown from the plane of an air bearing surface(ABS). There is a substrate 10 that in one embodiment is a bottom leadotherwise known as a bottom shield (Si) which may be a NiFe layer about2 microns thick that is formed by a conventional method on asubstructure (not shown). It should be understood that the substructuremay be comprised of a wafer made of AlTiC, for example.

A TMR stack is formed on the substrate 10 and in the exemplaryembodiment has a bottom spin valve configuration wherein a seed layer14, AFM layer 15, pinned layer 16, tunnel barrier layer 17, free layer18, and capping layer 19 are sequentially formed on the substrate. Theseed layer 14 is preferably a Ta/Ru composite but Ta, Ta/NiCr, Ta/Cu,Ta/Cr or other seed layer configurations may be employed, instead. Theseed layer 14 serves to promote a smooth and uniform grain structure inoverlying layers. Above the seed layer 14 is an AFM layer 15 used to pinthe magnetization direction of the overlying pinned layer 16, and inparticular, the outer portion or AP2 layer (not shown). The AFM layer 15has a thickness from 40 to 300 Angstroms and is preferably comprised ofIrMn. However, one of PtMn, NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, orMnPtPd may be employed as the AFM layer.

The pinned layer 16 preferably has a synthetic anti-parallel (SyAP)configuration represented by AP2/Ru/AP1 where a coupling layer made ofRu, Rh, or Ir, for example, is sandwiched between an AP2 layer and anAP1 layer (not shown). The AP2 layer which is also referred to as theouter pinned layer contacts the AFM layer 15 and may be made of CoFewith a composition of about 10 atomic % Fe and with a thickness of about10 to 50 Angstroms. The magnetic moment of the AP2 layer is pinned in adirection anti-parallel to the magnetic moment of the AP1 layer. Forexample, the AP2 layer may have a magnetic moment oriented along the“+x” direction while the AP1 layer has a magnetic moment in the “−x”direction. A slight difference in thickness between the AP2 and AP1layers produces a small net magnetic moment for the pinned layer 16along the easy axis direction of the TMR sensor to be patterned in alater step. Exchange coupling between the AP2 layer and the AP1 layer isfacilitated by a coupling layer that is preferably comprised of Ru witha thickness from 3 to 9 Angstroms. The AP1 layer is also referred to asthe inner pinned layer and may be a single layer or a composite layer.In one aspect, the AP1 layer is amorphous in order to provide a moreuniform surface on which to form the tunnel barrier layer 17. The AP1layer may be comprised of CoFeB, CoFe, or a composite thereof, and hasan upper surface that contacts the tunnel barrier layer 17.

In the exemplary embodiment that features a bottom spin valveconfiguration, the tunnel barrier layer 17 is preferably comprised ofMgO because a MgO tunnel barrier is known to provide a higher TMR ratiothan a TMR stack made with an Al₂O₃ or TiOx tunnel barrier. However, thepresent invention anticipates that the TMR stack may have a tunnelbarrier made of MgZnO, ZnO, Al₂O₃, TiOx, AITiO, HfOx, ZrOx, orcombinations of two or more of the aforementioned materials includingMgO.

In an embodiment where the tunnel barrier layer 17 is made of MgO, apreferred process is to DC sputter deposit a first Mg layer having athickness between 4 and 14 Angstroms on the pinned layer 16, and thenoxidize the Mg layer with a natural oxidation (NOX) process beforedepositing a second Mg layer with a thickness of 2 to 8 Angstroms on theoxidized first Mg layer as described in related U.S. Pat. No. 8,472,151.In one aspect, the tunnel barrier is considered as having a MgO/Mgconfiguration. The second Mg layer serves to protect the subsequentlydeposited free layer from oxidation. It is believed that excessiveoxygen accumulates at the top surface of the MgO layer as a result ofthe NOX process and this oxygen can oxidize a free layer that is formeddirectly on the MgO portion of the tunnel barrier layer. Note that theRA and MR ratio for the TMR sensor may be adjusted by varying thethickness of the two Mg layers in tunnel barrier layer 17 and by varyingthe natural oxidation time and pressure. Longer oxidation time and/orhigher oxygen pressure will form a thicker MgO layer and increase the RAvalue.

All layers in the TMR stack may be deposited in a DC sputtering chamberof a sputtering system such as an Anelva C-7100 sputter depositionsystem that includes ultra high vacuum DC magnetron sputter chamberswith multiple targets and at least one oxidation chamber. Typically, thesputter deposition process involves an argon sputter gas and a basepressure between 5×10⁻⁸ and 5×10⁻⁹ torr. A lower pressure enables moreuniform films to be deposited.

The NOX process may be performed in an oxidation chamber within thesputter deposition system by applying an oxygen pressure of 10⁻⁶ Torr to1 Torr for about 15 to 300 seconds. In the exemplary embodiment, noheating or cooling is applied to the oxidation chamber during the NOXprocess. Oxygen pressure between 1×10⁻⁶ and 1 Torr is preferred for anoxidation time mentioned above in order to achieve a RA in the range of0.5 to 5 ohm-um². A mixture of O₂ with other inert gases such as Ar, Kr,or Xe may also be used for better control of the oxidation process. Wehave found that the final RA uniformity (1σ) of 0.6 um circular devicesis more than 10% when a MgO tunnel barrier layer is rf-sputtered andless than 3% when the MgO tunnel barrier is formed by DC sputtering a Mglayer followed by a NOX process.

Returning to FIG. 1, an important feature of the present invention isthe free layer 18 formed on the tunnel barrier layer 17. In a firstembodiment illustrated in FIG. 2, the free layer 18 has a laminatedstructure represented by (CoFe/CoFeB)_(n), (CoFeM/CoFeB)_(n),(CoFeLM/CoFeB)_(n), (CoFe/CoFeBM)_(n), (CoFeM/CoFeBM)_(n), or(CoFeLM/CoFeBM)_(n) where n≧2, L and M are one of Ni, Ta, Ti, W, Zr, Hf,Tb, and Nb, and M is unequal to L. The CoFeB layer has a[Co_((100-x))Fe_(X)]_(100-Y)B_(Y) composition where x is from 0 to 100atomic % and y is between 10 and 40 atomic %. In a CoFeBM alloy, the Bcontent is preferably between 5 and 40 atomic %. In one example, M=Ni togive a CoNiFeB layer. The CoNiFeB layer may be formed by co-sputteringCoB and CoNiFe. CoB is preferably used in the co-sputtering process as ameans of adjusting λ.

In the exemplary embodiment where n=2, CoFe, CoFeM, or CoFeLM layers 18a 1, 18 a 2 alternate with boron containing layers 18 b 1, 18 b 2. WhenM, or L and M are selected from the group of Ta, Ti, W, Zr, Hf, Tb andNb, the content of M or L+M in the CoFeM alloy and CoFeLM alloy,respectively, is preferably less than 10 atomic %.

A first stack of layers is shown as 18 a 1/18 b 1 and a second stackformed on the first stack is designated 18 a 2/18 b 2 where layer 18 a 2contacts layer 18 b 1 and layer 18 b 2 is the uppermost layer. Note thata CoFe layer 18 a 1 contacts the underlying tunnel barrier layer 17 andthereby separates the CoFeBM or CoFeB layer 18 b 1 from the tunnelbarrier. The present invention also anticipates a free layer structurewith a plurality of “n” stacks of 18 a/18 b layers wherein n>2 and thetwo layers in the upper stack would be designated 18 an/18 bn (notshown) and the 18 a and 18 b layers are formed in alternating fashionbeginning with an 18 a 1 layer contacting the tunnel barrier layer 17.

Each 18 a layer (18 a 1 to 18 an) is comprised of CoFe, CoFeM, or CoFeLMand has a thickness between 2 and 8 Angstroms and each 18 b layer (18 b1 to 18 bn) is comprised of CoFeB or CoFeBM and has a thickness from 10to 80 Angstroms. Total thickness of free layer 18 is preferably lessthan 100 Angstroms. Note that the thickness of each of the CoFe, CoFeM,or CoFeLM layers is less than the thickness of each of the CoFeB orCoFeBM layers to maximize the MR ratio. In other words, MR ratiodecreases as the CoFe, CoFeM, or CoFeLM content increases in free layer18. According to the present invention, it is important that an 18 alayer contacts the tunnel barrier layer 17. Otherwise, a TMR structurein which a CoFeB or CoFeBM layer 18 b contacts a tunnel barrier wouldlead to a higher noise level and lower MR ratio. It is also important tocap or laminate 18 b layers with 18 a layers for the purpose of noisereduction.

In a second embodiment, the 18 b 1, 18 b 2 layers in an example wheren=2 are comprised of CoB and the CoB layer is preferably a lowmagnetostriction material Co_(S)B_(T) where T is from 5 to 30 atomic %and S+T=100 atomic %. Alternatively, one or more CoB layers may bereplaced by an alloy such as CoBM or CoBLM. Moreover, one or more of the18 a layers may be comprised of CoFe, a CoFeM alloy, or a CoFeLM alloyas in the first embodiment. When M, or L and M are selected from thegroup of Ta, Ti, W, Zr, Hf, Tb and Nb, the content of M or L+M in theCoFeM alloy and CoFeLM alloy, respectively, is preferably less than 10atomic %.

Thus, free layer configurations of the second embodiment are representedby (CoFe/CoB)_(n), (CoFe/CoBM)_(n), (CoFe/CoBLM)_(n), (CoFeM/CoB)_(n),(CoFeM/CoBM)_(n), (CoFeM/CoBLM)_(n), (CoFeLM/CoB)_(n),(CoFeLM/CoBM)_(n), and (CoFeLM/CoBLM)_(n) where n≧2, L and M are one ofNi, Ta, Ti, W, Zr, Hf, Tb, and Nb, and L is unequal to M. The B contentin the CoBM and CoBLM alloy is from about 5 to 30 atomic %.

Each of the 18 a layers (CoFe, CoFeM, or CoFeLM) has a thickness from 2to 8 Angstroms and each of the 18 b layers (CoB, CoBM, or CoBLM) has athickness from 10 to 80 Angstroms. Total thickness of the free layer 18is preferably less than 100 Angstroms.

Referring to FIG. 3, a third embodiment of the present invention isdepicted in which free layer 18 has a configuration represented by(CoFe/CoFeB)_(n)/CoFe, (CoFeM/CoFeB)_(n)/CoFeM, (CoFe/CoFeBM)_(n)/CoFe,(CoFeM/CoFeBM)_(n)/CoFeM, (CoFeLM/CoFeB)_(n)/CoFeLM, or(CoFeLM/CoFeBM)_(n)/CoFeLM where n is ≧1, and L and M are one of Ni, Ta,Ti, W, Zr, Hf, Tb, or Nb, and L is unequal to M. As in the firstembodiment shown in FIG. 2, the CoFe, CoFeM, or CoFeLM layers 18 aalternate with the boron containing layers 18 b, and a layer 18 acontacts the tunnel barrier layer 17. However, in this embodiment thetop layer is a non-boron containing layer. When n=1, a layer 18 a 1contacts the tunnel barrier layer, a layer 18 b 1 is formed on layer 18a 1, and layer 18 a 2 is the uppermost layer in the free layer 18 stack.When n=2, the uppermost layer is layer 18 a 3 rather than a boroncontaining layer. As a result, non-boron containing layers (i.e. 18 a 1,18 a 2, 18 a 3) alternate with boron containing layers (18 b 1, 18 b 2).When n>2, the top CoFe, CoFeM or CoFeLM layer may be designated as 18 anand the 18 a layers from bottom to top are in succession 18 a 1, 18 a 2,. . . 18 an and alternate with the 18 b layers that are formed frombottom to top in succession 18 b 1, 18 b 2, . . . 18 b(n−1). It is alsoimportant in this embodiment that the tunnel barrier layer 17 iscontacted by an 18 a layer and not an 18 b layer. Uppermost layer 18 ancontacts the capping layer (not shown).

In a fourth embodiment, the free layer 18 has a configurationrepresented by (CoFe/CoB)_(n)/CoFe, (CoFe/CoBM)_(n)/CoFe,(CoFe/CoBLM)_(n)/CoFe, (CoFeM/CoB)_(n)/CoFeM, (CoFeM/CoBM)_(n)/CoFeM,(CoFeM/CoBLM)_(n)/CoFeM, (CoFeLM/CoB)_(n)/CoFeLM,(CoFeLM/CoBM)_(n)/CoFeLM, or (CoFeLM/CoBLM)_(n)/CoFeLM where n≧1, L andM are one of Ni, Ta, Ti, W, Zr, Hf, Tb, and Nb, and L is unequal to M.The B content in the CoB, CoBM, or CoBLM alloy is from about 5 to 30atomic %.

In an example where n=1, a layer 18 a 1 contacts the tunnel barrierlayer 17, a layer 18 b 1 is formed on layer 18 a 1, and a layer 18 a 2is disposed on layer 18 b 1. When n>1, the top CoFe, CoFeM or CoFeLMlayer may be designated as 18 an and the 18 a layers are formed frombottom to top in succession 18 a 1, . . . 18 an. Likewise, the boroncontaining 18 b layers are formed from bottom to top in succession 18 b1, . . . 18 b(n−1).

After the free layer 18 is formed, a capping layer 19 is deposited onthe free layer. Capping layer 19 may be comprised of Ru, Ta, orcombinations thereof such as Ru/Ta/Ru. A Ru upper layer is typicallypreferred since Ru is resistant to oxidation, provides a good electricalconnection to an overlying top lead (top shield) formed in a subsequentstep, and serves as a CMP stop during a subsequent planarizationprocess.

Once the TMR stack is complete, the partially formed read head 1 may beannealed in a vacuum oven within the range of 240° C. to 340° C. with anapplied magnetic field of at least 2000 Oe, and preferably 8000 Oe forabout 2 to 10 hours to set the pinned layer and free layer magnetizationdirections. It should be understood that under certain conditions,depending upon the time and temperature involved in the anneal process,the tunnel barrier layer 17 may become a uniform MgO tunnel barrierlayer as unreacted oxygen diffuses into the adjacent Mg layer. Inanother embodiment, a two step anneal process as described previouslymay be employed.

Referring to FIG. 4, the TMR stack is patterned by following aconventional process sequence. For example, a photoresist layer 20 maybe coated on the capping layer 19. After the photoresist layer 20 ispatterned, a reactive ion etch (RIE), ion beam etch (IBE), or the likeis used to remove underlying layers in the TMR stack that are exposed byopenings in the photoresist layer. The etch process stops on the bottomshield 10 or between the bottom shield and a barrier layer (not shown)to give a TMR sensor with a top surface 19 a and sidewalls 21.

Referring to FIG. 5, an insulating layer 22 may be deposited along thesidewalls 21 of the TMR sensor. The photoresist layer 20 is then removedby a lift off process. A top lead otherwise known as a top shield 25 isthen deposited on the insulating layer 22 and top surface 19 a of theTMR sensor. Similar to the bottom shield 10, the top shield 25 may alsobe a NiFe layer about 2 microns thick. The TMR read head 1 may befurther comprised of a second gap layer (not shown) disposed on the topshield 25.

Comparative Example

An experiment was conducted to demonstrate the improved performanceachieved by implementing a free layer in a TMR sensor according to thepresent invention. A TMR stack of layers, hereafter referred to asSample A and shown in Table 1, was fabricated as a reference and iscomprised of a CoFe/CoB free layer wherein the lower Co₇₀Fe₃₀ layer is 5Angstroms thick and the upper Co₈₀B₂₀ layer is 48 Angstroms thick. Thisfree layer was disclosed in related U.S. Pat. No. 8,472,151. All samplesin the experiment have a seed/AFM/AP2/Ru/AP1/MgO/free layer/Ruconfiguration where AP2 and AP1 layers are comprised of CoFe, the seedlayer is Ta/Ru, and the AFM layer is IrMn. The MgO tunnel barrier wasformed by depositing a 7 Angstrom thick lower Mg layer that wassubjected to a NOX process before a 4 Angstrom thick upper Mg layer wasdeposited. Sample B comprises a free layer represented by (CoFe/CoB)₂according to the second embodiment of the present invention. Sample C(n=1) and Sample D (n=2) have a (CoFe/CoB)_(n)/CoFe free layer accordingto the fourth embodiment of the present invention.

The thicknesses in Angstroms of the other TMR layers are given inparentheses: Ta(20)/Ru(20) seed layer; IrMn (70) AFM layer;CoFe(25)Ru(7.5)CoFe(20) pinned layer; and Ru(50) capping layer. The TMRstack was formed on a NiFe shield and was annealed with a two stepprocess comprised of a first anneal at 250° C. for 3 hours and a secondanneal at 280° C. for 1.5 hours with an applied field of 8000 Oe toachieve high dR/R while maintaining good magnetic softness.

TABLE 1 Magnetic properties of MgO MTJs with Fe₇₀Co₃₀/Co₈₀B₂₀ based freelayers Sam- Free Layer Hc ple Composition Bs (Oe) Lambda RA dR/R AFeCo5/CoB48 0.60 4.6 1.20 × 10⁻⁶  1.8 60% B FeCo5/CoB24/ 0.68 5.9 8.2 ×10⁻⁷ 1.7 60% FeCo5/CoB24 C FeCo5/CoB50/ 0.63 5.3 2.0 × 10⁻⁶ 1.8 58%FeCo5 D FeCo5/CoB22/ 0.72 6.8 1.3 × 10⁻⁶ 1.8 58% FeCo5/CoB22/FeCo5

Sample B was formed by inserting a thin CoFe layer in the CoB layer ofSample A and resulted in a slight increase in He of about 1 Oe. Therewas little or no effect on Hc, λ, RA, and dR/R. For Sample C, a thinCoFe layer was added above the CoB layer in Sample A. Sample Drepresents insertion of a thin CoFe layer in the CoB layer in Sample Aand addition of a thin CoFe layer as the top layer in the free layerstack. A CoFe cap caused a slight decrease of about 2% in dR/R while λ,Hc, and RA are all comparable to the Sample A reference. All threeSamples B-D exhibited reduced noise with no signal loss (not shown)compared with Sample A which is a significant improvement over prior arttechnology.

Although not bound by theory, we believe that the noise reduction effectmay be attributed to a change in free layer microstructure, modificationof boron distribution in the CoB layer such that there is a lowerconcentration near the interface with CoFe layers, magnetic couplingbetween CoFe layers, or a combination of one or more of theaforementioned effects.

The free layers disclosed in the embodiments found herein may befabricated without additional cost since no new sputtering targets orsputter chambers are required. Furthermore, a low temperature annealprocess may be employed which is compatible with the processes formaking GMR sensors. Therefore, there is no change in process flow andrelated processes compared with current manufacturing schemes.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

We claim:
 1. A method of forming a sensor element in a magnetic device,comprising: (a) sequentially forming a stack of layers comprised of aseed layer, anti-ferromagnetic (AFM) layer, and a SyAP pinned layerhaving an AP2/coupling/AP1 configuration on a substrate wherein the AP2layer contacts said AFM layer; (b) forming a tunnel barrier having afirst surface that contacts the AP1 layer and a second surface oppositethe first surface; (c) forming a composite free layer on the tunnelbarrier, the composite free layer is comprised of: (1) at least oneCoFeB, CoFeBM, CoB, CoBM, or CoBLM layer having a first thickness andwhere L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb, L is unequalto M, and said at least one CoFeB, CoFeBM, CoB, CoBM, or CoBLM layer isformed in an alternating configuration with the plurality of CoFe,CoFeM, or CoFeLM layers; and (2) a plurality of CoFe, CoFeM, or CoFeLMlayers each having a second thickness less than the first thickness, andone of the CoFe, CoFeM, or CoFeLM layers contacts said second surface ofthe tunnel barrier; and (d) forming a capping layer on the compositefree layer.
 2. The method of claim 1 wherein the first thickness of eachof the at least one CoFeB, CoFeBM, CoB, CoBM, and CoBLM layers is about10 Angstroms to 80 Angstroms.
 3. The method of claim 1 wherein thesecond thickness is from about 2 to 8 Angstroms.
 4. The method of claim1 wherein the tunnel barrier is comprised of MgO, MgZnO, ZnO, Al₂O₃,TiOx, AlTiO, HfOx, ZrOx, or a combination of two or more of theaforementioned materials.
 5. The method of claim 1 wherein the tunnelbarrier is MgO and is formed by a process of DC sputter depositing afirst Mg layer, performing a natural oxidation process to yield a MgOlayer, and then DC sputter depositing a second Mg layer on said MgOlayer.
 6. The method of claim 1 further comprised of annealing thesensor element in a vacuum oven within a temperature range of about 240°C. to 340° C. with an applied magnetic field of 2000 Oe to 8000 Oe forabout 2 to 10 hours.
 7. The method of claim 1 further comprised of a twostep annealing process wherein a first anneal is performed at atemperature of about 250° C. for 3 hours, and a second anneal isperformed at a temperature of about 280° C. for 1.5 hours with anapplied field of 8000 Oe.